If no players have started playing yet, delete the entire round, deactivate the player and redraw a new round.
If players have started playing, you are so-out-of-luck. If it's just one or two people, run the groups with no-shows as three-player groups. Use the "adjust points" feature to adjust the points for the affected players.
If one of the no-shows is in a three-player group (making it a two-player group) I would consider using the new feature above to fiddle with the groups so the no-show is in a four-player group (making it a three-player group).
If this is a regular occurrence and you have a reasonably sized turnout (e.g. 35 or fewer people) I would strongly encourage you to do a roll call before you start the tournament. That way you can catch any players that didn't show up.